Saturation Monitoring with the RST Reservoir Saturation Tool- Disk 2

Saturation Monitoring With the RST Reservoir Saturation Tool

Bob Adolph The RST Reservoir Saturation Tool combines the logging capabilities of tra- Christian Stoller ditional methods for evaluating saturation in a tool slim enough to pass Houston, Texas, USA through tubing. Now saturation measurements can be made without killing Jerry Brady ARCO Alaska, Inc. the well to pull tubing and regardless of the well’s salinity. Anchorage, Alaska, USA Determining hydrocarbon and water satura- The Saturation Blues Charles Flaum tions behind casing plays a major role in In a newly drilled well, openhole resistivity Montrouge, France reservoir management. Saturation measure- logs are used to determine water and hydro- ments over time are useful for tracking reser- carbon saturations. But once the hole is Chuck Melcher voir depletion, planning workover and cased, saturation monitoring has to rely on Brad Roscoe enhanced recovery strategies, and diagnos- tools such as the TDT Dual-Burst Thermal Ridgefield, Connecticut, USA ing production problems such as water Decay Time tool or, for C/O logging, the influx and injection water breakthrough. GST Induced Gamma Ray Spectrometry Amal Vittachi Traditional methods of evaluating satura- Tool, which can “see” through casing. DeWayne Schnorr tion—thermal decay time logging1 and car- The Dual-Burst TDT tool looks at the rate Anchorage, Alaska bon/oxygen (C/O) logging2—are limited to of thermal neutron absorption, described by high-salinity and nontubing wells, respec- the capture cross section Σ of the formation, For help in preparation of this article, thanks to Darwin tively. The RST Reservoir Saturation Tool to infer water saturation (terms in bold are Ellis and Jeff Schweitzer, Schlumberger-Doll Research, overcomes these limitations by combining explained in “Gamma Ray Spectrometry at Ridgefield, Connecticut, USA; and Mohamed Watfa, Abu both methods in a tool slim enough to fit a Glance,” page 38). A high absorption rate Dhabi, U.A.E. 3 In this article, ELAN (Elemental Log Analysis), CNL through tubing. The RST tool eliminates indicates saline water, which contains chlo- (Compensated Neutron Log), Gradiomanometer, RST the need for killing the well and pulling tub- rine, a very efficient, abundant thermal-neu- (Reservoir Saturation Tool), GST (Induced Gamma Ray ing. This saves money, avoids reinvasion of tron absorber. A low absorption rate indi- Spectrometry Tool), Dual-Burst and TDT (Thermal Decay Time) are marks of Schlumberger. Macintosh is a mark of perforated intervals, and allows the well to cates fresh water or hydrocarbon. Apple Computer, Inc. VAX is a mark of Digital Equip- be observed under operating conditions The TDT technique provides good satura- ment Corporation. (next page). Moreover, it provides a log of tion measurements when formation water 1. Steinman DK, Adolph RA, Mahdavi M, Marienbach E, Preeg WE and Wraight PD: “Dual-Burst Thermal the borehole oil fraction, or oil holdup, salinity is high, constant and known. But oil Decay Time Logging Principles,” paper SPE 15437, even in horizontal wells. To understand the production from an increasing number of presented at the 61st SPE Annual Technical Confer- operation and versatility of the RST tool reservoirs is now maintained by water injec- ence and Exhibition, New Orleans, Louisiana, USA, October 5-8, 1986. requires an overview of existing saturation tion. This reduces or alters formation water 2. Woodhouse R and Kerr SA: “The Evaluation of Oil measurements and their physics.4 salinity, posing a problem for the TDT tool. Saturation Through Casing Using Carbon-Oxygen Logs,” paper SPE 17610, presented at the SPE Interna- tional Meeting on Petroleum Engineering, Tianjin, 3. Audah T and Chardac J-L: “Reservoir Fluid Monitoring Roscoe BA, Stoller C, Adolph RA, Boutemy Y, Cheese- China, November 1-4, 1988. Using Through-Tubing Carbon/Oxygen Tools,” Trans- borough JC III, Hall JS, McKeon DC, Pittman D, See- actions of the SPWLA 34th Annual Logging Sympo- man B and Thomas SR: “A New Through-Tubing Oil- sium, Calgary, Alberta, Canada, June 13-16, 1993, Saturation Measurement System,” paper SPE 21413, paper LL. presented at the SPE Middle East Oil Show, Bahrain, Stoller C, Scott HD, Plasek RE, Lucas AJ and Adolph November 16-19, 1991. RA: “Field Tests of a Slim Carbon/Oxygen Tool for 4. For more on nuclear logging: Reservoir Saturation Monitoring,” paper SPE 25375, Ellis DV: Well Logging for Earth Scientists. New York, presented at the SPE Asia Pacific Oil and Gas Confer- New York, USA: Elsevier, 1987. ence and Exhibition, Singapore, February 8-10, 1993. Tittman J: Geophysical Well Logging. Orlando, Scott HD, Stoller C, Roscoe BA, Plasek RE and Adolph Florida, USA: Academic Press, Inc., 1986. RA: “A New Compensated Through-Tubing Ellis D, Grau J, Schweitzer J and Hertzog R: “Basics of Carbon/Oxygen Tool for Use in Flowing Wells,” Nuclear Logging,” Oilfield Review 35, no. 3 (July Transactions of the SPWLA 32nd Annual Logging 1987): 4-10. Symposium, Midland, Texas, USA, June 16-19, 1991, paper MM.

January 1994 29 Near Carbon/ Far Carbon/ Volumetric In low-salinity water (less than 35,000 parts Oxygen Ratio Oxygen Ratio Analysis per million), the tool cannot accurately differentiate between oil and water, which Water Borehole Oil Sw have similar neutron capture cross sections. Shut-In Shut-In Holdup Shut-In Shut-In Oil When the salinity of the formation water Depth, ft Borehole Oil Sw Limestone is too low or unknown, C/O logging can be Flowing Flowing Holdup Flowing Flowing 0 1.0 0.1 0.6 -20 120 100 0 Dolomite used. C/O logging measures gamma rays emitted from inelastic neutron scattering to determine relative concentrations of carbon

X700 and oxygen in the formation. A high C/O ratio indicates oil-bearing formations; a low C/O ratio indicates water- or gas-bearing formations (next page, top). The major drawback to C/O logging tools has been their large diameters. Producing wells must be killed and production tubing removed to accommodate tools with diameters of nearly 4 in. [10 cm]. In addition, the tools have slow logging speeds and are more sensitive to borehole fluid than formation fluid, which affects the precision of the X800 saturation measurement.

As Easy as RST The RST tool directly addresses these short- comings and can perform either C/O or TDT logging (see “Logging the RST Tool in Prudhoe Bay,” page 32). It comes in two diameters—111/16 in. (RST-A) and 21/2 in. (RST-B)—and can be combined with other production logging tools (next page, bottom). The RST-A tool logs up to four X900 times faster than the GST tool. The RST-B nComparing RST logs run in the same Middle East well during shut-in (red) and flowing tool—the only C/O tool that can log flowing (blue) conditions. Production is from a vertical, 6-in. diameter, openhole completion. wells—makes passes at speeds comparable Tracks 1 and 2 show the carbon/oxygen logging ratio curves for both the near and to the GST tool. far detectors. Tracks 3 and 4 show the RST interpretation (borehole oil holdup and Both versions have two gamma ray detec- water saturation logs). Track 5 is the volumetric analysis. tors. In the RST-A tool, both detectors are on Both the near and far C/O ratios show a sharp increase at X848 ft, indicating an oil- water interface in the borehole. Above 850 ft, the C/O ratios from both detectors the tool axis, separated by neutron and increase steadily, showing the depths at which the oil is produced. gamma ray shielding. In the RST-B tool, the The borehole oil holdup during flowing indicates that most of the oil is produced from detectors are offset from the tool axis and the interval X728 to X750. The water saturation curves separate from X770 to X850 ft, shielded to enhance the near detector’s indicating that oil from the borehole reinvaded the formation while the well was shut- in. After the shut-in period, when the well flowed, the formation water flushed out the borehole sensitivity and the far detector’s oil, as shown by the increased water saturation and verified by the flowing borehole oil formation sensitivity. This allows the forma- holdup. The volumetric analysis from openhole ELAN Elemental Log Analysis interpre- tion oil saturation and borehole oil holdup tation indicates substantial oil saturation in the upper half of the reservoir. to be derived from the same RST-B C/O measurement. Because of size constraints, such detector shielding is not possible with the RST-A tool. An independent determi- nation of borehole fluid holdup is then needed, for example from the Gradioma- nometer tool run on the same logging suite or by logging shut-in. For both tools, the detector crystal is cerium-doped gadolinium oxyorthosilicate (GSO), one of a new generation of scintillation crystals that outperforms the sodium-

(continued on page 34)

30 Oilfield Review

nCombining the diagnostic capabilities ∑ Open Hole Carbon/Oxygen Carbon/Oxygen and of C/O logging and TDT logging with the RST-B tool in a Middle East observation Connate Water C/O Near Connate Water well. From left to right: Track 1 shows Ratio openhole fluid analysis. Track 2 shows Movable Oil Mixed Water Injection Water C/O Far C/O logs from the near and far detectors and oil holdup. Track 3 shows the fluid Residual Oil 0Ratio 0.5 Remaining Oil Remaining Oil Depth, ft analysis based on C/O logging. Track 4 Pore Volume Borehole Oil Holdup Pore Volume∑ From RST Pore Volume shows the log of Σ. Track 5 shows a com- % c.u. Σ 50 p.u. 0 -20 120 50 p.u. 0 30 10 50 p.u. 0 bined C/O and fluid analysis. The observation well contained water in the borehole over the entire interval, so the oil holdup value was set to 0. The C/O X150 fluid analysis, used to distinguish between oil and water, shows only a small oil depletion from X190 ft to X250 ft and a large oil depletion from X140 ft to X170 ft.

By itself, however, C/O logging cannot differentiate between injection and con-

X200 nate water. To accomplish this, a log of Σ

is used in conjunction with the C/O logs.

The Σ log can differentiate between oil

and salt water but not oil and fresh water. The revised interpretation indicates for- X250 mation water from X170 to X210 ft and a water injection breakthrough from X140 to X170 ft. Interpretation based on Σ alone would have identified the water injection breakthrough as oil.

RST-A Sonde RST-B Sonde 111/16 in. 21/2 in.

Side View a a a aa aElectronics aa Photomultiplier tube a a a a

GSO detector a a a a (far) a a a a Electronics

Photomultiplier tube a a a a GSO detector (near) a a a a Shielding a a a a

aaaa

Neutron aagenerator aa

Top View aa a a a aa a a a a a a a a a a a a Far a aaa a a aaa a a a a a a a a Near a a aaa aaa

nThe dual-detector RST-A and RST-B tools.

January 1994 31 Logging the RST Tool in Prudhoe Bay

Prudhoe Bay, the largest oil field in North Amer- Connate Water (OWC) at X1250 ft and the original gas-oil contact ica, contained 20 billion stock tank barrels when Borehole Oil (GOC) at X1050 ft. Holdup Flowing Injection Water it was discovered in 1968.1 Most of the hydrocar- and Gas By 1992, production rates fell, indicating 50p.u. 0 bons are in the Ivishak Sandstone. The main Gas severe reservoir depletion. The well was produc- recovery methods used in Prudhoe Bay are grav- Oil ing 3750 B/D of fluids with a 94% water cut. Pro- Gamma Ray ity drainage, gas injection, aquifer influx, water- Porosity duction was 220 BOPD and the gas/oil ratio Depth, ft 0 GAPI 100 flooding and miscible gas flooding. Over time, 40 p.u. 0 (GOR) was 1115 ft3 of gas per barrel of oil. The X1000 the expansion of gas above and water below the RST tool was run to evaluate hydrocarbon distri- oil column has produced mobile oil lenses that bution and locate fluid contacts. are elusive to tap. The salinity of injected water C/O data measured with the well flowing were is low, creating conditions suited to C/O logging combined with CNL Compensated Neutron Log with the RST tool.2 data as input to the ELAN Elemental Log Analysis As the Prudhoe Bay field matures, increasing program (right track). The resulting fluid analysis gas and water production is exceeding the capac- confirmed oil depletion over both perforated ity of surface handling facilities, thus limiting oil intervals and identified the current GOC at X1110 X1100 production. No pipeline exists for the gas, so it ft. The borehole oil holdup log showed oil pro- GOC must be reinjected. Wells with declining oil production from perforations above X1170 ft and was duction and increasing gas and water production used to identify the present OWC at that depth. are typically worked over or produced intermit- The lower perforations were not producing any tently. Extending the life and economic viability oil. After these were plugged, the well produced of these marginal wells relies on reducing gas OWC 300 BOPD of oil with negligible water cut. and water production and maximizing oil produc- Perfs tion by producing bypassed oil zones. Locating Bypassed Oil X1200 RST data were used to reduce water cut in a In early 1992, ARCO drilled and perforated a BP well with a 250-ft [76-m] oil-bearing sand- sidetrack well in an area of Prudhoe Bay under- stone interval (right). Openhole logs from 1983 Perfs going waterflooding. Less than six months later, (not shown) marked the original oil-water contact production was 90% water with less than 200 BOPD, as expected. The original perforations extended from X415 to X440 ft (next page). C/O 1. Shirzadi SG and Lawal AS: “Multidisciplinary Approach for Tar- nReducing water cut. The left track shows the gamma geting New Wells in the Prudhoe Bay Field,” paper SPE 26093, ray and borehole oil holdup logs measured with the well logging measurements were made In the shut-in presented at the SPE Western Regional Meeting, Anchorage, producing. The borehole oil holdup curve shows no oil well with three different tools—the RST tool and Alaska, USA, May 26-28, 1993. production below 1170 ft. 2. Vittachi A and Schnorr D: “Reservoir Oil Saturation Monitoring The right track is an ELAN Elemental Log Analysis two sondes from other service companies. Through Casing With New Carbon/Oxygen Measurements,” output based on openhole and RST fluid data. It indi- The RST results confirmed depletion over the Transactions of the SPWLA 34th Annual Logging Symposium, Calgary, Alberta, Canada, June 13-16, 1993, paper HHH. cates oil depletion below 1170 ft. After the lower perfo- perforated interval (Tracks 2 and 3). Effects of the rated interval was plugged, production increased from Dupree JH and Cunningham AB: “The Application of miscible gas flood sweep are apparent through- Carbon/Oxygen Logging Technology to the Ivishak Sandstone, 220 BOPD oil with 94% water cut to 300 BOPD oil with Prudhoe Bay, Alaska,” paper SPE 19615, presented at the 64th negligible water cut. out the reservoir. The total inelastic count rate SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, October 8-11, 1989. ratio of the near and far detectors indicates quali- tatively the presence of gas in the reservoir. In

32 Oilfield Review Openhole Data CRRA Connate Water Sand 4.0 2.6 Gas Gas Bound Water Saturation Oil Oil Shale Depth, ft Gamma Ray Saturation Porosity Lithology 0GAPI 150 0 140p.u. 00 10

GOC X100 nFinding bypassed oil zones. ARCO ran three C/O logging tools with the well shut-in over the same interval in search of bypassed oil. Track 1 includes the gamma ray, count-rate ratio between near and far detectors (CRRA) and the borehole oil holdup curves. Track 2 X200 shows the RST oil saturation and gas saturation B curves. Track 3 shows the RST fluid analysis. Track 4 is openhole log data. The RST data identified Zone A as the best zone for perforating. Interpreta- tion based on logs from a A non-RST C/O logging tool identified Zone B for per- X300 foration, which the RST data show to be mostly water. Zone B was perforated first, producing 200 BOPD with more than 95% water cut in a matter of weeks as a result of water coning into the per- foration. When Zone A was perforated, oil production increased to an X400 average of 600 BOPD with a reduced water cut of 90%. All water production

Perfs was identified as from Zone B. addition, differences between the openhole fluid 75% water cut. Production declined to 200 BOPD analysis and the RST fluid analysis were with more than 95% water cut in a matter of assumed to be gas. weeks. The decline prompted ARCO to perforate One potential bypassed zone, A, was identified Zone A, commingling production from earlier per- from X280 to X290 ft. A second zone, B, based on forations. Production increased to an average of the openhole logs and a C/O log from another ser- 600 BOPD and the water cut decreased to 90%. vice company, was proposed from X220 to X230 Subsequent production logs confirm that Zone A ft. The RST log shows Zone B to contain more gas is producing oil and gas and Zone B is producing and water than Zone A. all of the water with some oil. After assessing the openhole logs and the three C/O logs, ARCO decided to perforate Zone B. The initial production was 1000 BOPD with a

January 1994 33 iodide [NaI] crystal conventionally used for 100 gamma ray detection (see “New Scintilla- tion Detectors,” page 35). Several properties of GSO allow for a smaller diameter detector crystal than if NaI were used, and hence a smaller diameter tool. 80

Modes of Operation Flexibility is a key advantage of the RST C tool. It operates in three modes that can be changed in real time while logging: Counts •inelastic-capture mode 60 •capture-sigma mode •sigma mode. sec

Inelastic-capture mode: The inelastic-cap- µ ture mode offers C/O measurements for

determining saturations when the formation Time, water salinity is unknown, varying or too 40

low for TDT logging. In addition to C/O log- Counts ging, thermal neutron capture gamma-ray Net inelastic spectra are recorded after the neutron burst. B Elemental yields from these spectra provide lithology, porosity and apparent water salinity information. 20 In the inelastic-capture mode, each measurement cycle contains one neutron burst and three timing gates for collecting spectra Energy

(right). The first gate measures the total A Counts gamma ray spectrum during the neutron burst, which contains both inelastic and burst Neutron 0 capture spectra. The second gate measures Energy an early capture spectrum following the neutron burst, which is used to subtract the nTiming for the inelastic-capture mode of the RST tool. Timing gate A capture background from the previous records inelastic spectra during the neutron burst. Timing gates B and inelastic spectrum, yielding the net inelastic C record capture spectra after the burst. The net inelastic spectrum is formed by subtracting a fraction of the early capture spectrum mea- spectrum. The C/O ratio from the net inelas- sured in gate B from the spectrum measured in gate A. tic spectrum is used to determine saturation. The third gate measures a capture spectrum mation as in the inelastic-capture mode. Interpretation after the neutron burst, which is used to Total count rate measurements are used to The conversion of C/O ratios to saturations determine formation lithology. For example, determine Σ for the formation and the bore- relies on an extensive data base designed to the ratio of silicon to calcium is used to hole. Unlike the inelastic-capture mode, measure RST tool response to a variety of distinguish silicates from carbonates. Log- each measurement cycle in the capture- borehole environments. The RST-A and ging passes are made at 60 to 100 ft/hr [18 sigma mode contains two neutron pulses RST-B tools have been logged in a wide to 30 m/hr]. —a short one and a long one. Total count variety of conditions at Schlumberger’s Capture-sigma mode: The capture-sigma rates collected during and after the short Environmental Effects Calibration Facility in mode is used to determine lithology and the burst are used to determine the borehole Houston, Texas, USA. For each combina- capture cross section Σ in the same logging fluid Σ; total count rates collected after the tion of lithology, casing and cement at the pass. It simultaneously records capture longer burst are used to determine the for- facility, four C/O ratio measurements were gamma ray spectra and total capture gamma mation Σ. Logging is usually performed at made to cover the four combinations of oil ray count rates. Elemental yields from the 600 ft/hr [180 m/hr]. and water in the borehole and formation: capture spectra can provide lithology, Sigma mode: The sigma mode is used water-water, water-oil, oil-water and oil-oil. porosity and apparent water salinity infor- when the salinity of the formation water is The resulting measurements represent the high enough for TDT logging. It provides largest C/O characterization data base for capture cross-section data in a fast pass—up any nuclear tool in the oil field today. Simi- to 1800 ft/hr [550 m/hr]. Although the tim- larly, the largest data base of capture-sigma ing sequence mimics the capture-sigma measurements was also acquired. Different mode, only decay-time data and a pulse height spectrum, for calibrating the tool gain, are recorded.

34 Oilfield Review New Scintillation Detectors

Running the RST tool through tubing would not be possible without a scintillation crystal called cerium-doped gadolinium oxyorthosilicate (GSO), one of several new crystals finding its way to the oil patch. Developing these crystals for borehole detectors requires coupling a knowledge of nuclear physics with sophisticated crystal-growing techniques.

Comparison of Scintillation Crystals Crystal Density Effective Refractive Relative Energy Light Flash Rugged Unaffected Dewar g/cm3 Atomic Number Z Index Light Flash Resolution Decay Time2 By Moisture System Intensity @662keV1, % ns Required NaI(Tl) 3.67 51 1.85 100 6.5 230 No No No BGO 7.13 75 2.15 15 9.3 300 Yes Yes Yes GSO 6.71 59 1.85 20 8.0 56 and 600 No Yes No LSO 7.40 66 1.82 75 10 40 Yes Yes Depends on application

1. Energy resolution refers to the commonly used relative “full width at half maximum” for the 137Cesium gamma ray peak at 662 keV. Sharply defined spectral peaks indicate good energy resolution. 2. Decay time is the time constant for the exponential decay of the light flash. Faster decay times are preferred.

What Makes a Great Crystal? resolution, and makes the detector more sensi- The New Detectors Scintillation detectors are so named because tive to low-energy gamma rays. If at all possible, For nearly 45 years, thallium-doped sodium they generate flashes of light when struck by the light-flash intensity and duration should be iodide (NaI) has been the gamma ray detector of gamma rays (for details on how scintillator crys- relatively unaffected by temperature change. This choice for nuclear logging. It is widely used to tals work, see “Gamma Ray Spectrometry at a eliminates the need for temperature control hard- determine formation density and chemical com- Glance,” page 38).1 Most of the properties that ware downhole. position, salinity through casing, and mineralogy. make a crystal desirable for logging are those C/O logging and thermal decay time logging NaI excels in intensity of light flash and tempera- that maximize the intensity of these flashes and expose the crystal to high instantaneous gamma ture stability. Although new crystals literally do the number of counts.2 In addition, crystals ray fluxes during or immediately after the neutron not outshine NaI, they do bring increased detec- should be rugged, not cracking on impact, and burst. The light flashes must have a short enough tion efficiency, greater ruggedness, reduced sen- unaffected by moisture. duration to avoid “piling up” on one another, sitivity to humidity, and the ability to handle A crystal of high density and high atomic num- which produces false pulse amplitudes and much higher counting rates without pileup (see ber provides more opportunity for interaction with counting rates. “Comparison of Scintillation Crystals,” above). incident gamma rays, maximizing count rates. Once a light flash has been generated, the Bismuth germanate (BGO) was first manufac- Crystal volume also affects performance. A big- goal is to get as much light as possible to the tured commercially in the early 1970s, providing ger crystal intercepts more gamma rays, thereby photomultiplier tube (PMT). To prevent internal higher counting rates than NaI that result in bet- increasing count rates. It also decreases the absorption of light, the crystal should be trans- probability that a gamma ray will escape the parent to the light it generates. Because light 1. For a discussion of scintillation spectrometry: crystal after one or two scatterings. generated by the crystal must eventually pass Knoll GF: Radiation Detection and Measurement. New York, New York, USA: John Wiley and Sons, 1989. A high light-flash intensity produces large volt- through the window of the photomultiplier tube, Birks JB and Fry D: The Theory and Practice of Scintillation ages that are easily measured, improving energy the index of refraction of the crystal and window Counting. London, England: Pergamon Press, 1964. 2. Melcher CL, Schweitzer JS, Manente RA and Peterson CA: should be similar to maximize light transmission. “Applications of Single Crystals in Oil Well Logging,” Journal of The photocathode of the PMT must be highly Crystal Growth 109 (1991): 37-42. responsive to the wavelength of the crystal light to maximize the number of photoelectrons it ejects.

January 1994 35 n Comparison of a combinations of formation and borehole GSO crystal used in Σ the RST tool and salinities were used to generate over 1000 the larger NaI crys- measurements for each tool. tal used in the GST The C/O data base is used to calibrate the tool. GSO’s higher C/O ratio (COR) model to a particular log- density and higher ging environment. The COR model relates atomic number allow it to function measured C/O ratios to the porosity, oil sat- in the small diame- uration, lithology, oil density, oil holdup, ter RST tool. borehole diameter and casing weight and diameter. Once calibrated, the model is inverted to solve for fluid volumes,5 and these values are further refined using alpha processing, described below. The oil volumes are then converted to saturations.

Alpha Processing: Windows Versus Yields Schlumberger’s traditional approach to pro- ter precision (right). Its use in the Hostile Natural BGO vs. NaI cessing C/O logging data, the C/Oyields Gamma Ray Sonde (HNGS) offers the choice of approach, is to determine the carbon and Uncertainties BGO NaI oxygen yields from the measured inelastic faster logging or improved statistical precision in Th 2.82 ppm 4.63 ppm spectrum, and then use their ratio to deter- estimates of thorium, uranium and potassium. mine oil volume, then saturation. The U 1.51 ppm 2.61 ppm BGO’s high detection efficiency is also used in a C/Oyields approach provides good accuracy 33/8-in. neutron-induced spectrometry tool.3 In K 0.51% 0.74% but reduced precision.6 Advantages of this general, higher count rates and improved spec- Th = 12 ppm, U = 6 ppm, K = 2% method are ease of use and interpretation. Because the contribution of carbon to the tral peak-to-Compton ratios more than compen- nQuantifying BGO’s statistical advantage. total spectrum is relatively small, however, sate for the main limitation of BGO, low intrinsic Uncertainties for thorium (Th), uranium (U) and potassium (K) outputs are pre- good statistics require long measurement resolution. GSO and BGO crystals generally give sented for 1-sec accumulations in a stan- times or slow logging speeds. To shorten better precision than similarly sized NaI crystals dard shale. Standard deviations of Th, U measurement times without sacrificing pre- and K concentration estimates drop by 30 cision, a “windows” approach has been in estimating element concentrations from neu- to 40 % when a BGO detector is used tron-induced capture gamma ray spectra. The instead of a NaI detector. adopted. Windows are sections of the spectrum most influenced by changes in carbon poor temperature response of BGO, however, lutetium oxyorthosilicate (LSO), a cousin of GSO, and oxygen and least influenced by requires the use of a Dewar flask. BGO can also promises to be the best scintillator for many changes in other elements (next page, left). experience pileup problems. applications.6 Although still in development, this The ratio of the gamma ray counts from the The RST tool is the first commercial logging scintillator combines high counting efficiency “carbon window” to counts in the ”oxygen tool to use the GSO crystal, which was developed with a light-flash intensity nearly that of NaI and window” is then used to determine a more precise saturation. by the Hitachi Chemical Co., Ltd. in the early five times that of BGO. It is unaffected by mois- The C/O approach provides good 4 windows 1980s. The high density and high atomic number ture, is rugged and can handle higher counting precision but poor accuracy. It improves the of GSO make for a smaller diameter detector than rates free of pileup than other scintillators under statistics of the measurements because the NaI, and hence a smaller diameter tool (top).5 consideration for borehole use. The light gener- total counts in the windows are high. But The high density also improves the crystal’s ated by an LSO crystal, similar in wavelength interpretation is more difficult. For example, ability to detect gamma rays, particularly high- to that of NaI, works well with existing photo- in a clean, water-bearing sand and a water- filled borehole, the C/O ratio would be energy ones. cathode materials. yields zero, whereas the C/Owindows ratio is non- GSO has a detection efficiency nearly as high Competing crystals are breaking the near zero. The ratio produced by the windows as BGO’s but can operate at higher temperatures monopoly held by NaI for over four decades, method must be calibrated with ratios mea- without a bulky Dewar flask. It produces signifi- though the price is high. The pair of GSO scintil- cantly less pulse pileup during the neutron burst. lators in the RST tool costs about $10,000 at pre- 3. Jacobson LA, Beals R, Wyatt, DF Jr and Hrametz A: “Response Because the light output from GSO is lower than sent. But an expanding market is likely to result Characterization of an Induced Gamma Spectrometry Tool Using a Bismuth Germanate Scintillator,” Transactions of the for NaI, the RST tool employs a newly designed, in improved production methods and reduced SPWLA 32nd Annual Logging Symposium, Midland, Texas, sensitive PMT. prices. These new scintillators are sure to make USA, June 16-19, 1991, paper LL. 4. Takagi K and Fukazawa T: “Cerium-activated GD2SiO5 Single A new member of the scintillation detector further inroads on NaI’s territory. Crystal research Crystal Scintillator,” Applied Physics Letters 42 (1983): 43-45. family was developed two year ago by scientists and development are continuing and new crystals 5. Melcher CL, Schweitzer JS, Utsu R and Akiyama S: “Scintillation Properties of GSO,” IEEE Transactions of Nuclear Science 37, at Schlumberger-Doll Research, Ridgefield, Con- with special properties may be added to this no. 2 (April 1990): 161-164. necticut, USA. They found that cerium-doped expanding list of exotic detectors.—JT, TAL 6. Melcher CL and Schweitzer JS: “A Promising New Scintillator: Cerium-Doped Lutetium Oxyorthosilicate,” Nuclear Instruments and Methods in Physics Research A 314 (1992): 212-214.

36 Oilfield Review sured under known conditions. Ideally, the windows-based C/O tool would be logged first in a known water-bearing zone to determine a zero carbon value, then logged in a zone of known oil saturation to obtain a second calibration point. Alpha processing for the RST tool combines the accuracy of the yields ratio with the precision of the windows ratio to obtain saturation results in the minimum time. It calculates the volume of oil (the product of porosity and oil saturation) from the C/Owindows method—the windows oil volume, and the volume of oil from the C/Oyields method—the yields oil volume. The difference between these volumes is tracked and used to adjust the windows oil volume. Alpha processing is applied over a specified number of levels, known as the alpha filter length.

nThe RST Job Planner software for planning RST jobs on the Macintosh. The field engi- neer or client specifies the many input parameters, including RST tool type, lithology, saturation precision and alpha processing level, to calculate corresponding logging speeds with the RST tool. CDV stands for oil gravity. Logging speeds are reported for two levels of confidence, 68% and 95%. The use of alpha processing increases logging Carbon Oxygen speeds by improving measurement statistics. Carbon Planning the RST Job Future of the RST Tool With nuclear tools like the RST tool, careful With the RST tool’s accuracy, precision and Counts job planning can significantly affect the pre- versatility for saturation monitoring cision—and the cost—of the final answer. unmatched in the oil field, applications for Oxygen To facilitate planning an RST job, a software the tool continue to grow. The RST tool has program called RST Job Planner is used to recently provided holdup measurements in determine the logging speed and number of horizontal and through-tubing wells. A passes needed to produce results according porosity measurement determined from a 02 468to clients’ specifications. Versions of RST Job ratio of the count rates in the near and far Energy, MeV Planner can run on VAX computers at the detectors will soon be added to the sigma nInelastic spectra measured with the field logging interpretation center (FLIC) or mode. Software is being developed that will far detector of the RST tool in an oil on a Macintosh computer. The Macintosh convert between RST and TDT logs, and water tank. Shaded areas indicate version is available to clients. enhancing the value of RST logs in Σ-moni- the carbon and oxygen windows used to improve measurement statistics. RST Job Planner requires a variety of input toring programs. In addition, the RST tool’s parameters, including tool type, lithology, response to gas is being studied. These porosity, oil gravity (referred to as carbon ongoing projects, plus experimental use of density value—CDV), casing size and the RST tool in uncharacterized environ- weight, borehole diameter, number of levels ments by innovative field engineers and for alpha processing and number of levels of clients, promise an exciting future for this vertical averaging. In addition, clients spec- newest saturation monitoring device. —TAL ify the precision, or standard deviation of

the answer, in terms of a percent saturation. 5. Oil volume is the volume of formation taken up by oil. The output consists of station times and It can be expressed as porosity times oil saturation. logging speeds in a table showing two levels 6. Accuracy refers to how close the mean value of a of confidence,7 68% and 95% (above). This measurement is to the true value. Consider a speedometer that always registers 10 miles per hour allows the client to weigh precision needs (mph). It is inaccurate when the car is traveling 60 against logging costs. Higher precision mph but accurate when the car travels 10 mph. requires slower logging speeds. The fastest Precision refers to the spread of individual measurements from the mean. A precise system produces logging option, however, may not always be results with very little spread. The broken speedometer the most cost-effective for the client. While is very precise, since it always gives the same mea- the RST-B tool typically logs at less than half surement of 10 mph. 7. A precision of ± one standard deviation corresponds the rate of the RST-A, it can minimize the to 68% confidence; ± two standard deviations corre- time a well is off-line and avoid problems spond to 95% confidence and ± three standard devia- associated with reinvasion from shut-in. tions correspond to 99.9% confidence.

January 1994 37 Gamma Ray Spectrometry at a Glance

The interactions and measurements associated with gamma ray spectrometry may be confusing to nonexperts in nuclear logging. Simply put, gamma ray spectrometry measures gamma rays (counts) in time and gamma ray energies. Some elements emit gamma rays naturally; others can be bombarded with neutrons to induce gamma ray emissions. Each element produces characteristic gamma rays of specific energies. Moreover, the number of characteristic gamma rays produced is proportional to the abundance of the element. Naturally occurring and induced gamma rays may be counted and sorted according to energy. This produces a gamma ray spectrum that can be processed, or decoded, to identify the elements and their concentrations.

Downhole Accelerator Inelastic Neutron Scattering In inelastic neutron n 3 Ion source scattering, the neutron bounces off the nucleus, but excites it into quickly giving off what are n n called inelastic gamma Filament Magnet Target rays. The measurement of gamma ray energies The starting point for induced gamma ray from inelastic neutron 1 spectrometry is the generation of neutrons. Like scattering yields the its predecessors, the RST tool uses a downhole relative concentrations accelerator to emit pulses of high-energy of carbon and oxygen, neutrons into the formation. This device creates which are then used to 14-million electron volt (MeV) neutrons by determine water Inelastic saturation. accelerating deuterium ions into a tritium target. gamma rays

Elastic Neutron Scattering

Neutron Absorption

Slow Excited neutron nucleus

n The neutrons primarily interact with formation 2 nuclei in three ways: In elastic neutron-scattering, the neutron Capture bounces off the bombarded nucleus without gamma ray exciting or destablizing it. With each elastic interaction, the neutron loses energy. Hydrogen, In neutron absorption, the nucleus absorbs the neutron and becomes 4 with the mass of its nucleus equal to that of a excited, typically emitting capture gamma rays. Neutron absorption, or neutron, is very good at slowing down neutrons. neutron capture, is most common after a neutron has been slowed by Hence, how efficiently a formation slows down elastic and inelastic interactions to thermal energies of about 0.025 eV. neutrons generally indicates the abundance of The measurement of capture gamma ray energies is used to estimate the hydrogen. Because hydrogen is most abundant in abundances of elements most likely to capture a neutron—silicon, calcium, pore fluids, neutron slowdown indicates porosity. chlorine, hydrogen, sulfur, iron, titanium and gadolinium.

38 Oilfield Review Photomultiplier Tube

Gamma ray

Scintillation Window material Photoelectron Photocathode surface Short burst Long burst Counts

Collector 0 500 1000 1500 Time, µsec

A decrease in the production rate of capture 5 A gamma ray is detected when its interactions with the detector crystal create gamma rays over time, as measured with the 7 electrons and holes that excite the crystal into generating flashes of light TDT tool, is proportional to the absorption rate of (scintillations). The intensity of these scintillations is related to the energy of thermal neutrons. This decay is basically the bombarding gamma ray. Most gamma rays lose some of their energy on exponential and accounts for both the absorption the way to the detector because they scatter randomly, create new particles and diffusion of neutrons in the formation. The or disappear. slope of a semilog plot of gamma ray counts The light flashes pass through a window at one end of the crystal and Σ versus time yields the capture cross section of fall on the photocathode surface of a photomultiplier tube, liberating the formation. Expressed in capture units, the electrons via the photoelectric effect. The tube amplifies the electronic capture cross section expresses the probability charges about 200,000 times and provides a current signal large enough to that a neutron passing through a cross-sectional be analyzed by downhole electronics. area of a material will be captured.

Oxygen Silicon Tool background Calcium Iron Carbon Counts

0 20Time, µsec 100 Neutron Neutron burst burst

InelasticEarly Late Inelastic 12345678 capture capture Energy, MeV

A pulsed neutron source and “time-gated” detectors As with any other nuclear logging tool, the gamma ray spectra collected by 6 solve the difficulty of distinguishing gamma rays of 8 the RST tool must be processed to identify the elements that contributed to nearly the same energy from two different interactions. the spectra and their abundances. This is possible because each element Inelastic gamma rays occur during the neutron pulse; produces a characteristic set of gamma ray peaks. capture gamma rays occur later, after the neutrons have A library of standard elemental spectra is used to determine the been slowed down. A typical timing cycle takes individual elemental contributions. Each standard elemental spectrum advantage of this by scheduling the first measurement represents the response of a tool to a particular element. A computer gate during the neutron pulse and subsequent determines the linear combination of these elemental standards that best measurement gates at later times. Which interactions, fits the measured spectrum and calculates the elemental yields. A yield is and hence, which elements contribute to the spectra, the fractional contribution of an element to the total observed spectrum, depend largely on the sequence and duration of pulsed after subtracting off the capture background. It is proportional to the neutron bursts and timing gates. abundance of the element. 39